The ice sheet shapes the continent

Almost every landscape in northern North America was made by ice. The Great Lakes, Long Island, Cape Cod, the course of the Ohio River, the thousands of small lakes across Minnesota and Ontario — all of it is the trace of an ice cap that lay here and was gone only twelve thousand years ago.

How big

At its greatest the Laurentide ice sheet covered nearly all of Canada and reached south into Ohio and New York. In places it was over three kilometres thick. Together it locked up enough water to lower the world's oceans by around sixty metres — which is why dry land existed between Siberia and Alaska at the same time.

Timeline

2.6 million years ago

the ice ages begin; ice builds and melts in cycles

26,000 years ago

the last glacial maximum; the ice stands furthest south

20,000

the ice begins to retreat

14,000

ice-free corridors and coastal routes open southward

13,000

Lake Agassiz gathers in front of the ice margin

12,900

the Younger Dryas: the climate falls back into cold for a thousand years

11,700

the Holocene begins; the ice is essentially gone from the United States

8,200

Agassiz finally drains to the sea; one last cold snap

today

the land is still rising around Hudson Bay, a centimetre a year

What the ice did

An ice cap is not a fixed block; it flows slowly outward under its own weight. As it goes it scrapes loose everything not firmly fixed and carries it along. Where the ice gouged, hollows were left that later filled with water — the Great Lakes are such hollows. Where the ice stopped, it dumped its load in a ridge of gravel and stone. Long Island and Cape Cod are such ridges.

Between those extremes the ice left till: a mixture of clay, sand and stone, finely ground and well mixed. It is some of the best farmland in the world, and it is why the American grain belt lies exactly where the ice ended.

The land remembers the weight

Three kilometres of ice weighs enough that the crust gave way and sank into the slowly flowing layer beneath. When the ice melted, the land began to rise again — and it still is. The Hudson Bay region lifts about a centimetre a year, twelve thousand years after the weight went. The same thing happens in Scandinavia, for the same reason.

Why it concerns us

People reached the Americas while the ice was still there, and their route was decided by where it was not. The water the ice held made Beringia dry. The water it released may have thrown the climate back into cold. And the soil it left behind decided where a continent could feed itself.